In recent machine tools, there is known one provided with a triaxially driving mechanism which moves a main shaft portion (spindle portion), to which a machine tool is mounted, in X-, Y- and Z-axis directions and a rotationally driving mechanism for rotating a holding portion such as table for holding a work to be worked (workpiece) around a C- and/or A-axis (for example, refer to Patent Document 1 mentioned hereunder).
In such machine tool, generally, the triaxially driving mechanism is driven by a linear motor, and the rotationally driving mechanism is driven by a hollow motor. Particularly, the rotationally driving mechanism extremely speedily starts to operate by the direct driving of the hollow motor, and moreover, it is not necessary for a mechanism for rotationally driving the C-axis and A-axis to be provided with any transmission member such as pulley and/or reduction mechanism, thus achieving advantageous effect.
Further, in such rotationally driving mechanism, it is necessary to appropriately rotate the holding member such as table holding the workpiece to be worked around the C- and/or A-axis, and because of this reason, the holding member is mounted to a bed or like of the machine tool by means of bearing.    Patent Document 1: Japanese Patent Application Laid-open Publication No. 2004-130468